Ultrasound diagnostic equipment has long been used in the medical field to detect the flow of blood. Conventionally, the velocity of the blood is sensed by taking advantage of the Doppler frequency shift that occurs when the ultrasonic energy engages the moving blood. Conventional ultrasonic flow detectors generally utilize one of two transducer arrangements. One arrangement utilizes a single transducer operating in a pulsed mode. A transmitter is used to excite the transducer to transmit a brief pulse of ultrasonic energy, and then the transmitter is turned off. During the off-time and for the period prior to the next transmit pulse, the detector circuitry senses any signal produced on the transducer from the return reflected energy.
An alternative arrangement utilizes dual-transducers operating in a continuous wave mode. The transmitter operates by continuously sending ultrasonic energy from one of the transducers, while the return reflected energy is being simultaneously sensed by the second transducer. The two transducers effectively operate independent of each other. The continuous wave approach with dual-transducers requires a more complex transducer design and inherently has sensitivity limitations relating to the distance between the transducers and the area of reflection within the patient's body. Generally, the transducer elements are side-by-side pointing along parallel beam paths, so the transmit energy beam does not line up perfectly with the reflected energy beam. As such, reception is limited to the incidental scattering of the transmit beam. In the past, attempts have been made to overcome this limitation by lensing the acoustic beams to produce a beam cross-over point at some distance from the transducers. While this produces a good result as long as the area of reflection within the body is at the depth of penetration at which the beams are focused, reception outside of this limited area is not good.
While the single transducer operating in a pulsed mode as first discussed is generally a more expensive design to manufacture, it does overcome some of the problems associated with using dual-transducers with focused beams. This is because the transmitted energy and the received reflected energy travel along the same straight line path. Even with a single transducer operating in the pulse mode, the transducer will not work equally well at all distances from the transducer because the pulse rate used limits the reception window available due to the fixed speed of the acoustic wave traveling through the body.
In conventional blood flow detector designs, the transducer units are manufactured with one of three techniques. The first technique has the transducer clamped in position to a protective face plate using mounting screws or rings. In addition to protecting the transducer, the face plate can be provided with a thickness equal to a desired odd number of quarter-wave lengths of the acoustic output signal of the transducer for impedance matching and tuning. Alternatively, a second technique bonds the transducer to the face plate with an adhesive compound. The third technique has the protective face plate for the transducer being formed from the adhesive itself applied directly to the transducer. In each of these designs, the active transducer may or ay not be backed with an absorptive or reflective substance. In any event, the active element is rigidly attached to the face late and any flexure of the transducer or face plate may damage the transducer which is usually made of a brittle ceramic. Furthermore, mechanical impact, thermal variations and aging of the bonding agents used may cause the transducer to detach. Thermal variations or mechanical stress can cause the Q factor of the transducer to vary, and hence change its resonant frequency. In addition to these problems since the transducer is mechanically strained and restrained, either by the clamping or the adhesive used, the process of mounting the transducer to the protective face plate affects the operation of the transducer.
Another disadvantage experienced with the assembly of conventional transducer units using adhesive is a high rate of rejection due to the formation of micro-bubbles in the bond layer. The bubbles have an adverse impact on the ultrasonic energy passing through the bond layer.
It will therefor be appreciated that there is a significant need for an ultrasonic fluid flow detector which avoids these disadvantages. The flow detector should be inexpensive to manufacture and have a high manufacturing yield rate. The detector should be very sensitive, have a good signal-to-noise ratio, and have good spacial resolution and definition. The present invention avoids these disadvantages and fulfills these needs, and further provides other related advantages.